I know we've done it before but a recent thread made me wonder about membership. According to Jim, we know of at least six members not including Harry, McCoy or Vadderung. That leaves six we hadn't met prior to Changes (who we may have met since of course). We know some are wizards and some are "key allies". Yet all that turned up wore grey cloaks and held staves (or things that looked like staves *Vadderung*)
1. Ebenezar (duh)
2. Vadderung (duh)
3. Harry (duh) -
these are locked in4. Listens-to-Wind (Injun Joe). Arguably Ebenezar's best friend, and the only Council member apart from Eb who didn't vote that there was no Black Council at the end of Changes.
5. The Gatekeeper. Despite voting that there is no conspiracy, he is old and wise and seems to be very much against the bigger enemies. Seems like a good choice, and friendly with Eb.
6. Anastasia Luccio. Would be angry after Morgan's death and frustrated that the Council wasn't officially doing anything.
Likely candidates7. Martha Liberty. She did vote that there is no Black Council, officially. She also doesn't seem to be the type to break the rules. Yet she is friends with Eb, and the right type of candidate.
8. The Merlin. Often cited as an unexpected member, he has no reason to be a part of the Gray Council. No more than the Queen of England joining US parliament i.e. they each have their own team already. However, if he were the type to do a sort of Bruce Wayne double life, maybe this is how he would do it.
9. Carlos Ramirez. Often people think he might have joined in, but the latest books make him out to be the Merlin's man. He also probably isn't that well connected to McCoy. Yet he did notice the existence of the Black Council and named them Black Hats, so he is a maybe.
These are the less likely guesses10. Mavra. Would be an interesting choice. She is a powerful Wizard and combatant. She might be using the GC to get her own ends, but she might also resent the Black Council. And it would make life difficult for Harry having her as an ally. Not only that, she is canny and resourceful and might have excellent reasons to want to Red Court dead (which she helped achieve). However, Eb hates vampires particularly White Court. If he somehow got over his issue with her being a vampire, then perhaps they might have common ground in getting rid of the White Court.
11. Sir Baron Gentleman John Marcone. Gosh he has a lot of titles. I wonder what his end game is...
This would be a WILD reveal. Some of you might say "but he wasn't a Wizard yet" except he quite possibly was by this stage. Perhaps Vadderung recruited him, as he would have the connection to bring him in. However, I doubt Eb would like him any more than Harry.
These are the wild guesses12. Unknown. Possibly Lea or (better choice), the Erlking.
13. Unknown. Possibly Eldest Gruff.
The problem guesses. Also - honourable mention: River Shoulders (probably too big and doesn't need a wizard staff, but still would be fun and loyal).
The issue with the final two is that Lea is clearly not in the Gray Council, as all thirteen members (including Harry, Eb and Vadderung) are at Chichen Itza in Changes, and Eb and Vadderung bring the other 10. However the problem with having Eldest Gruff as a member is he would need to be balanced out by a Winter Fae surely (for the symmetry if not the politics), and Lea seems to be his counterpart...and yet she almost certainly is not a member. The Erlking is a good foil to Vadderung and they seem to work together a bit, so I can see him being a part of the GC. He normally doesn't wield a staff but he could I am sure, and we know he has powerful magic.
You could address the above Lea/Eldest Gruff issue by saying there is more than 13 Gray Council members, which is possible. But Jim seemed to imply in his 2010 Bitten Books interview that there was only 13.
2010 Bitten by Books Q&A:
#136 “I am also highly intrigued by the Gray Council showing up as 12 in number.”
Thirteen, actually: Harry was already there.
“So we found out who 2 of them were, i’d like to know how many of the rest we already know and how many of them haven’t been formally introduced to us yet.”
Hang on, counting… You already know six of the folks who showed up at Chichen Itza, excluding McCoy and Vadderung. 
